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The best argument for ignoring global warming has been that there are better ways to spend money: instead of devoting billions to curb carbon emissions and reduce the impact on sea levels in 2050, we could spend the resources developing a vaccine for AIDS or providing universal health care to all Americans.

In essence, the dangers of climate change appeared distant and uncertain, while the costs of curbing greenhouse gases were immediate and appeared substantial.

But all across the country, states and local governments have chipped away at those arguments for delay — actually, pretty much demolished them — by showing that there are myriad small steps we can take that significantly curb carbon emissions and that are easily affordable.

A leader of that effort has been Portland, earnestly green even when it is wintry gray. In 1993, the city adopted a plan to curb greenhouse gases, and it is bearing remarkable fruit: local greenhouse gas emissions are back down to 1990 levels, while nationally they are up 16 percent. And instead of damaging its economy, Portland has boomed.

This month Portland took an important additional step, by adopting a renewable-fuel standard. Beginning July 1, 2007, all diesel sold for vehicles in the city will have to be at least a 5 percent biodiesel blend. And all gasoline will have to contain at least 10 percent ethanol. This measure is not a magic bullet, but it has a negligible cost and is one more example of the creative thinking at local levels that is curbing greenhouse gases without breaking the bank.

Portland joins several states — Minnesota, Washington, Hawaii, Montana, Iowa, Louisiana and Missouri — that have passed similar laws.

“We aren’t confused into thinking that we can change the world,” said Randy Leonard, the city commissioner who sponsored the ordinance, which passed unanimously. “We’re just hoping that we can establish a template to reduce our dependence on foreign oil and promote economic development and reduce greenhouse gases. Then we can send a message.

“We get that we’re an asterisk to the larger issue of oil dependence and global warming,” he added. “But we also think that there isn’t an energy policy in the state or nation that makes sense, and here there is something we can do that does make sense.”

The 5 percent biodiesel works in all diesel vehicles, without any loss of acceleration. The bio- component sometimes comes from discarded restaurant grease; vehicles using it smell faintly of French fries.

The benefit of the 10 percent ethanol requirement is less clear because U.S. ethanol sometimes takes nearly as much petroleum to make (in fuel to run tractors to harvest corn, for example) as it saves. But increasingly we’ll probably be making ethanol from switch grass or sugar cane in ways that are more likely to save petroleum.

Portland customers may end up paying slightly more for gas and diesel because of the new regulation, but city planners say that any increase will be minimal — perhaps a penny or two per gallon.

“This will reduce total emissions by 1 percent,” said Michael Armstrong, a city environmental expert. “It sounds tiny, but if you think that the Kyoto standard is a 7 percent reduction [below 1990 levels], and this one simple move would be 1 percent, then it seems more doable.”

The city is also seeking to ensure that all city government units use power from wind or solar sources by 2010. Portland has changed streetlamps to use low-power bulbs, built bicycle trails to encourage commuting on bike or on foot, and greatly expanded mass transit.

It’s true that the risks of climate change are uncertain, but when encountering other kinds of dangers — like Iran apparently trying to develop nuclear weapons — we don’t shrug and say there’s no point in doing anything because of the uncertainties. The risks of warming are potentially enormous — imagine much of the east coast under water — and our obligation to protect our planet is not just technical but also moral.

So it’s time to abandon the old self-defeating notion that curbing greenhouse gases is too costly to be effective. Portland and other localities are showing that there’s plenty we can do inexpensively, at least in the early phases — if we don’t mind rush-hour traffic smelling of French fries.

I almost didn’t write this column, because with the Middle East in flames it’s obvious that climate change is not the most important topic of the day. But it could be the most important issue of this century.

A version of this op-ed appeared in print on July 30, 2006, on page WK13 of the New York edition with the headline: Another Small Step For Earth.